The present invention relates to a receiver device for a multi-element ultrasonic probe echograph and an echograph equipped in this way. It more particularly relates to the field of ultrasonic medical diagnosis. Piezoelectric transceiver means are known from the prior art. These devices or piezoelectric probes make it possible to transmit ultrasonic waves when piezoelectric crystals are excited by electric pulses at a predetermined transmission frequency. The transmitted waves are propagated in the medium of which it is desired to form an image. They are partly reflected by acoustic impedance differences between the nonhomogeneous media encountered. The reflections are called echos. They are pressure waves which the piezoelectric probe translates into electric pulses called response echo signals. As the ultrasonic wave has a certain propagation velocity in the medium, each echo is received after transmission at a time which is dependent on the distance from the obstacle which reflects it. For carrying out automatic scans of the inside of a body by means of ultrasonics, it has been proposed to transmit ultrasonic waves by multi-element probes. A plurality of piezoelectric transducer elements are distributed in such probes. On transmission, the ultrasonic beam is formed by exciting each transducer element in phase relationship with its neighbours.
In a first construction called a linear scanning probe, the latter comprises N adjacent elements. At a given time, the ultrasonic beam is produced by exciting P adjacent elements taken from among the N elements of the probe. By applying predetermined time delays which vary from one element to the next, it is possible to focus the ultrasonic energy of the beam at a predetermined point, called the examination focus. By displacing the addressing of electrical signals relative to the series of P adjacent elements, it is possible to carry out a linear scan of the examination plane.
In a second construction called a sector scanning probe, the time delay law applied to each transducer element of the probe is such that the focus obtained can be displaced in an angular manner about the probe axis.
On reception, each echo signal received is transmitted to the processing device of the signal across a regulatable delay line by an electronic control circuit. In order to obtain high operating dynamics it is necessary for each delay line to be able to supply delays which are as short as possible and delays which exceed one second. At all times, the delay laws applied to the delay line elements must be such that the various signals transmitted to the processing device correspond at a given time to the same echo reflected by the obstacle.
It is known to use charge coupled devices or CCD's for obtaining these delay lines. In such a device, any signal supplied to the input is progressively transmitted at a given input analysis speed and at a passage rate which are determined by a clock signal able to vary in wide frequency ranges, which determine the dynamics and therefore the characteristics of the ultrasonic beam received.
To obtain different delays for each delay line channel, the clock frequency is diversified as a function of the reception angle and consequently the transducer element in question. The regulation of the clock frequencies must take place rapidly and with a given precision. The necessary electronic circuits for such a construction considerably increase the cost of the examination equipment. This is particularly the case when frequency regulation takes place in a synthesizer by means of an oscillator controlled by the VCO voltage. Another problem encountered with delay line service devices is the thermal drift. Finally, it is also necessary to compensate the voltage dependence.